fallbackswitch: protect src pad stream lock using Cond

Should prevent stream and State deadlocks, see https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/issues/202

Fix #202
Hopefully fix #192 as well.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1330>

utils/fallbackswitch/src/fallbackswitch/imp.rs

@@ -14,7 +14,7 @@ use gst::{debug, log, trace};
 
 use once_cell::sync::Lazy;
 
-use parking_lot::{Mutex, MutexGuard};
+use parking_lot::{Condvar, Mutex, MutexGuard};
 use std::sync::atomic::{AtomicU32, Ordering};
 
 const PROP_PRIORITY: &str = "priority";
@@ -87,6 +87,10 @@ struct State {
 
     timeout_running_time: Option<gst::ClockTime>,
     timeout_clock_id: Option<gst::ClockId>,
+
+    /// If the src pad is currently busy. Should be checked and waited on using `src_busy_cond`
+    /// before calling anything requiring the stream lock.
+    src_busy: bool,
 }
 
 impl Default for State {
@@ -102,6 +106,8 @@ impl Default for State {
 
             timeout_running_time: None,
             timeout_clock_id: None,
+
+            src_busy: false,
         }
     }
 }
@@ -420,6 +426,7 @@ impl SinkState {
 #[derive(Debug)]
 pub struct FallbackSwitch {
     state: Mutex<State>,
+    src_busy_cond: Condvar,
     settings: Mutex<Settings>,
 
     // Separated from the rest of the `state` because it can be
@@ -952,11 +959,13 @@ impl FallbackSwitch {
         }
 
         if switched_pad {
+            self.with_src_busy(|| {
                 let _ = pad.push_event(gst::event::Reconfigure::new());
                 pad.sticky_events_foreach(|event| {
                     self.src_pad.push_event(event.clone());
                     std::ops::ControlFlow::Continue(gst::EventForeachAction::Keep)
                 });
+            });
 
             self.obj().notify(PROP_ACTIVE_PAD);
         }
@@ -987,11 +996,13 @@ impl FallbackSwitch {
 
             let out_gap_event = builder.build();
 
+            self.with_src_busy(|| {
                 self.src_pad.push_event(out_gap_event);
+            });
 
             Ok(gst::FlowSuccess::Ok)
         } else {
-            self.src_pad.push(buffer)
+            self.with_src_busy(|| self.src_pad.push(buffer))
         }
     }
 
@@ -1114,15 +1125,18 @@ impl FallbackSwitch {
         drop(state);
 
         if fwd_sticky {
+            self.with_src_busy(|| {
                 let _ = pad.push_event(gst::event::Reconfigure::new());
                 pad.sticky_events_foreach(|event| {
                     self.src_pad.push_event(event.clone());
                     std::ops::ControlFlow::Continue(gst::EventForeachAction::Keep)
                 });
+            });
 
             self.obj().notify(PROP_ACTIVE_PAD);
         }
-        self.src_pad.push_event(event)
+
+        self.with_src_busy(|| self.src_pad.push_event(event))
     }
 
     fn sink_query(&self, pad: &super::FallbackSwitchSinkPad, query: &mut gst::QueryRef) -> bool {
@@ -1237,6 +1251,34 @@ impl FallbackSwitch {
 
         false
     }
+
+    /// Wait until src_busy is not set and set it, execute
+    /// the closure, then unset it again and notify its Cond.
+    ///
+    /// The State lock is taken while modifying src_busy,
+    /// but not while executing the closure.
+    fn with_src_busy<F, R>(&self, func: F) -> R
+    where
+        F: FnOnce() -> R,
+    {
+        {
+            let mut state = self.state.lock();
+            while state.src_busy {
+                self.src_busy_cond.wait(&mut state);
+            }
+            state.src_busy = true;
+        }
+
+        let ret = func();
+
+        {
+            let mut state = self.state.lock();
+            state.src_busy = false;
+            self.src_busy_cond.notify_one();
+        }
+
+        ret
+    }
 }
 
 #[glib::object_subclass]
@@ -1260,6 +1302,7 @@ impl ObjectSubclass for FallbackSwitch {
 
         Self {
             state: Mutex::new(State::default()),
+            src_busy_cond: Condvar::default(),
             settings: Mutex::new(Settings::default()),
             active_sinkpad: Mutex::new(None),
             src_pad: srcpad,